1. Field of the Invention
The invention relates generally to monitoring partial discharges, and more particularly to systems and methods for monitoring partial discharge within the stator of a remotely located electric motor.
2. Related Art
Oil production often requires the use of artificial lift systems to recover oil and other well fluids from wells. These artificial lift systems may include, for example, electric submersible pump (ESP) systems and subsea boosting systems. Because of the high cost of these systems (including installing, operating, maintaining, reworking, etc.) and the inaccessibility of the systems, it is important that the systems are as reliable as possible.
One of the system components that impacts their overall reliability is the electrical insulation in the system. Insulation is provided between the turns of the stator windings, between the windings of each phase (motors typically utilize three-phase power), and between the windings and the stator core and housing. Poor insulation quality may degrade the performance of the motor or even cause the system to fail. It is therefore important to be able to determine whether the quality of the electrical insulation is degraded (e.g., through damage during manufacture or through extended operation of the system).
Partial discharge testing can provide one measure of insulation quality in a motor. “Partial discharge” is a partial dielectric breakdown of an insulator. This breakdown occurs in small isolated areas in the insulator, often at weak points or defects such as small gas bubbles, voids or inclusions in the insulator. Partial discharge is seen most often in high voltage applications where potential levels are high and non-uniform electric fields generate accentuated electrical stresses. Any small inclusion or void in the high potential area of the insulation system is more likely to breakdown, creating a discharge in the void. These small discharges span across the void, and do not discharge across the entire insulating material. Consequently, it is only a partial discharge. Partial discharges cause insulation to deteriorate, making further partial discharges more likely.
As noted above, partial discharge can be an indicator of insulation damage or deterioration. Partial discharge testing can be performed offline (when the motor is not operating) or online (during operation of the motor). There are a variety of techniques for sensing partial discharge, including radio-frequency and acoustic sensing. It is preferable to position the sensing devices as closely as possible to the insulation that is of concern in order to maximize their sensing capabilities. In surface motors, there may be a great deal of flexibility in the positioning of the sensors and the connection of these sensors to monitoring equipment, whether the testing is performed offline or online. In motors that are used in ESP's and other remotely located systems, however, the physical (e.g., size) constraints on the motors and the extremely harsh environments in which they operate can make the prospect of testing much more complicated.
In addition to the difficulty of implementing partial discharge testing in motors used in artificial lift applications, various other factors have conventionally discouraged this testing. For example, downhole and subsea motors typically operate at lower voltages than comparable surface motors, so partial discharge is mitigated. Further, the high pressures that are present are communicated to the oil within the motor and may reduce partial discharge to the point that it is negligible. Still further, the lifespan of a downhole or subsea motor is typically expected to be relatively short (e.g., five years), so insulation degradation due to partial discharge has not traditionally been a concern in these motors.
Because of the complexity of implementing partial discharge testing in motors for downhole, subsea and other remote and inaccessible applications, and the fact that partial discharge has not traditionally been a concern in these motors, testing of the motors conventionally is not performed after they are installed and operating.